1. Field of the Invention
The present invention relates to a developer carrying member for developing an electrostatic image on an image bearing member, and a developing apparatus provided with such member. Such developer carrying member and developing apparatus are advantageously employed in an image forming apparatus such as a copying apparatus or a printer utilizing an electrophotographic process or an electrostatic recording process, and in a process cartridge detachably attachable on such apparatus.
2. Related Background Art
FIG. 9 is a schematic view showing a typical example of an image forming apparatus. The image forming apparatus of this example is a copying apparatus or a printer utilizing an electrophotographic process of transfer type.
A drum-shaped electrophotographic photosensitive member 1 (hereinafter referred to as “photosensitive drum”) is provided as an image bearing member and is rotated in a direction R1 with a predetermined peripheral speed (process speed), and image forming process including charging, image exposure, development, transfer and cleaning is applied to the photosensitive drum 1.
More specifically, a surface of the rotated photosensitive drum 1 is uniformly charged to a predetermined polarity and a predetermined potential by a primary charger (charging roller) 2. In this example, there will be explained a case of employing a photosensitive drum 1 to be charged negatively.
Then the charged surface is subjected to an image exposure by image exposure means 3 constituting image information writing means and constituted for example of a projection exposure apparatus for an unrepresented original image or a scanning exposure apparatus with an imagewise modulated laser beam, whereby the charged potential in an exposed light portion is attenuated to form, on the surface of the photosensitive drum 1, an electrostatic latent image corresponding the exposed image information.
Such electrostatic latent image is rendered visible in succession, at a developing position N4, as a transferrable developer image (toner image or visible image) by a developing apparatus 4.
The toner image thus formed on the photosensitive drum 1 is transferred, at a transfer position N5, onto a transfer material (transfer paper) 5 by transfer means 5.
The transfer means in the present example is of contact transfer type utilizing a roller-shaped contact transfer charger 5 (hereinafter referred to as “transfer roller”).
The transfer roller 5 is constituted for example of a metal core and an elastic layer of a medium resistance formed around such metal core, and is pressed to the photosensitive drum 1 under a predetermined pressure, against the elasticity of the elastic layer, thereby forming a transfer position (transfer nip portion) N5. It is rotated in a direction R5, same as the rotating direction of the photosensitive drum 1, and with a peripheral speed approximately same as that of the photosensitive drum 1.
A transfer material P is fed from a feeding unit 16, and is advanced to the transfer position N5 under a timing control by unrepresented registration rollers provided in front of the transfer position N5.
More specifically, the registration rollers advance the transfer material P in such a timing that a leading end of the transfer material P arrives at the transfer position N5 when a leading end of a toner image area formed on the surface of the rotating photosensitive drum 1 reaches the transfer position N5.
The transfer material P supplied to the transfer position N5 is pinched and conveyed, with a surface thereof in contact with the photosensitive drum 1, through the transfer position N5. Also during a period from the arrival of the leading end of the transfer material P at the transfer position N5 to the passing of the trailing end of the transfer material out of the transfer position N5, a predetermined transfer bias voltage of a positive polarity is applied to the metal core of the transfer roller 5 from an unrepresented transfer bias source.
In the course of pinched conveying of the transfer material P through the transfer position N5, the toner image on the photosensitive drum 1 is transferred in succession onto the transfer material P, by a function of a transferring electric field formed by the transfer roller 5 functioning as a contact transfer charger and by a pressure at the transfer position N5.
The transfer material P, after emerging from the transfer position N5, is separated from the surface of the photosensitive drum 1 and is conveyed to a fixing device 9 in which the transferred toner image is fixed as a permanent image on the surface of the transfer material P, whereupon a formed image (copy or print) is discharged.
After the separation of the transfer material P, the surface of the photosensitive drum 1 is cleaned by a cleaner 10 constituting cleaning means, for eliminating deposited contamination such as remaining toner and paper dust, and is repeatedly used for image formation.
As a developing apparatus 4 to be employed in such image forming apparatus, there has been proposed and commercialized a dry one-component developing apparatus in various types. An example is an apparatus utilizing an impression (contact) development. Such impression development, not requiring a magnetic material, has various advantages such as enabling to simplify and compactize the apparatus and enabling a color image formation with non-magnetic toners.
FIG. 10 shows a developing apparatus utilizing impression development.
In the impression development, since the development is executed by pressing or contacting a surface of a developer carrying member with an electrostatic latent image, it is necessary to employ, as the developer carrying member, a developing roller 101 having elasticity and conductivity.
For achieving image formation with a predetermined density by the developing roller 101, a certain surface roughness has been required in order to carry a large amount of the developer. For obtaining a predetermined surface roughness, a layer containing insulating particles formed by an urethane resin or an acrylic resin is provided as an outermost layer 101b, but a resin layer is provided on the insulating particles in order to regulate the surface roughness, so that the insulating particles do not protrude on the surface of the developing roller 101.
Also for obtaining a known developing electrode effect or a known bias effect at the development, it is possible to form a conductive layer in the outermost layer 101b of the developing roller or in the vicinity of the outermost layer 101b, and to apply a bias voltage if necessary.
Also a charge provision to a developer (toner) 105 is achieved by a frictional charging between the developing roller 101 and a developing blade 102 which regulates a toner amount on the surface thereof thereby forming a toner layer.
However, in the impression development employing the aforementioned developing roller 101, there is experienced a drawback of a decrease of frictional charging ability on the toner when the toner is deteriorated with an increase in the number of image formations, thereby resulting in a fog formed by toner deposition in a solid white background.